This invention relates to the field of solar energy and, more particularly, to an improved solar energy collecting apparatus.
The idea of collecting energy from the sun's rays and converting it to useful thermal or other energy is quite old. In the past decade, however, the increasing cost of energy and recognition of the limited supply of fossil fuel have led to the development of innumerable schemes for solar energy collection. One type of solar energy collection device which has been developed in many forms is the so-called "evacuated tube solar collector". An early example of this type of device is disclosed in the U.S. Pat. No. 980,505 which issued in 1911. Other examples are set forth in the U.S. Pat. Nos. 1,946,184; 2,133,649; 3,125,091; and 3,227,153.
A simple form of an evacuated-tube solar collector consists of a fluid-carrying pipe and a hollow cylindrical transparent glass tube which surrounds the pipe and is spaced therefrom. A solar energy absorbing medium is typically coupled to the pipe, the simplest form of absorbing medium being a black coating applied to the pipe's outer surface. The space between the pipe and the glass enclosure is evacuated to leave a partial vacuum therein. In operation, solar energy passes through the glass and is absorbed by the black pipe. Fluid is circulated through the pipe to remove the heat for use as desired. Wavelength selective absorber coatings can minimize radiation heat losses. The vacuum region minimizes heat loss from the pipe that would otherwise occur by convection and conduction. An alternate known configuration of a vacuum tube solar collector is the "thermos bottle" type. In this collector a concentric pair of nested glass tubes is employed, the ends being sealed separately on one end and annularly joined at the other, with the resultant annular volume in the space between being evacuated. The outer tube is left transparent and the inner tube has a solar absorbing coating applied to the outside surface.
In a typical evacuated-tube solar collection installation, a plurality of side-by-side evacuated tube units are employed in conjunction with a plumbing system which circulates the heat transport fluid (e.g. water or air) through the pipes contained in the glass tubes. Various designs have been set forth which place the evacuated tube collectors at the foci of light concentrators so that the total light flux impinging upon the absorber is increased. This technique reduces the absorbing surface area in relation to the radiation collection area, in order to reduce heat losses per unit of collection area. For example, a pair of opposing concave or flat mirrorized structures can be positioned adjacent and above each evacuated tube collector, forming a vee, with the tube receiver at the apex. In this manner, solar radiation that would normally hit the region between the spaced evacuated-tube collectors can be directed, with reasonable precision, toward the absorber of an evacuated-tube collector. This can result in improved efficiency due to lowered heat losses, and in reduced cost, as less expensive reflectors are substituted for more expensive evacuated tubes. Imaging or nonimaging reflectors can be employed to derive these advantages. An example of an imaging reflector optically coupled to an evacuated tube absorber is the sun tracking parabolic trough. In this configuration, the absorber is precisely located at the foci, whose location depends upon the particular reflector design used. Although concentration is frequently desirable, in certain instances it is not, in which case other types of reflectors, such as involutes of the absorbers, can be employed.
It is thus well recognized that the combination of reflectors and evacuated tube collectors can provide a desirable solar heat collection system. However, such systems can tend to be large and bulky, and expensive to fabricate and use. To insure the cost-effectiveness of an installation, the solar collection system should not be unduly expensive to manufacture, install, or maintain. An object of the present invention is to provide a solar collection apparatus which meets these criteria while also providing efficient and reliable operation.